The loss of well integrity in oil and gas and CO2 injection wells provokes leaks that potentially pollute underground water reservoirs and the surrounding environment. The present publication reviews the existing literature investigating the loss of well integrity due to damage development in the cement sheath, focusing on qualitative and mainly quantitative information regarding cracks, effective permeability, and leak flows. Methods applied for leak detection on-site are reviewed, and the difficulties of these methods in providing quantitative results are highlighted. The outputs of laboratory experiments and computer simulations, considered essential to complement on-site measurements, are also reported. The review of the existing literature shows that for most of the damaged cement sheaths the observed crack widths range between 1 and 500 µm, the permeability ranges from 10−17 to 10−12 m2, and the leak rates range between 10 and 10 000 mL/min for gas leaks and between 1 and 1000 mL/min for oil leaks.